Estimation of labile sulfide in iron-sulfur proteins

Fluorescent probes for tracking the transfer of iron-sulfur cluster and other metal cofactors in biosynthetic reaction pathways

Iron-sulfur (Fe-S) clusters are protein cofactors that are constructed and delivered to target proteins by elaborate biosynthetic machinery. Mechanistic insights into these processes have been limited by the lack of sensitive probes for tracking Fe-S cluster synthesis and transfer reactions. Here we present fusion protein- and intein-based fluorescent labeling strategies that can probe Fe-S cluster binding. The fluorescence is sensitive to different cluster types ([2Fe-2S] and [4Fe-4S] clusters), ligand environments ([2Fe-2S] clusters on Rieske, ferredoxin (Fdx), and glutaredoxin), and cluster oxidation states. The power of this approach is highlighted with an extreme example in which the kinetics of Fe-S cluster transfer reactions are monitored between two Fdx molecules that have identical Fe-S spectroscopic properties. This exchange reaction between labeled and unlabeled Fdx is catalyzed by dithiothreitol (DTT), a result that was confirmed by mass spectrometry. DTT likely functions in a ligand substitution reaction that generates a [2Fe-2S]-DTT species, which can transfer the cluster to either labeled or unlabeled Fdx. The ability to monitor this challenging cluster exchange reaction indicates that real-time Fe-S cluster incorporation can be tracked for a specific labeled protein in multicomponent assays that include several unlabeled Fe-S binding proteins or other chromophores. Such advanced kinetic experiments are required to untangle the intricate networks of transfer pathways and the factors affecting flux through branch points. High sensitivity and suitability with high-throughput methodology are additional benefits of this approach. We anticipate that this cluster detection methodology will transform the study of Fe-S cluster pathways and potentially other metal cofactor biosynthetic pathways.

Novel, highly specific N-demethylases enable bacteria to live on caffeine and related purine alkaloids

The molecular basis for the ability of bacteria to live on caffeine as a sole carbon and nitrogen source is unknown. Pseudomonas putida CBB5, which grows on several purine alkaloids, metabolizes caffeine and related methylxanthines via sequential N-demethylation to xanthine. Metabolism of caffeine by CBB5 was previously attributed to one broad-specificity methylxanthine N-demethylase composed of two subunits, NdmA and NdmB. Here, we report that NdmA and NdmB are actually two independent Rieske nonheme iron monooxygenases with N(1)- and N(3)-specific N-demethylation activity, respectively. Activity for both enzymes is dependent on electron transfer from NADH via a redox-center-dense Rieske reductase, NdmD. NdmD itself is a novel protein with one Rieske [2Fe-2S] cluster, one plant-type [2Fe-2S] cluster, and one flavin mononucleotide (FMN) per enzyme. All ndm genes are located in a 13.2-kb genomic DNA fragment which also contained a formaldehyde dehydrogenase. ndmA, ndmB, and ndmD were cloned as His(6) fusion genes, expressed in Escherichia coli, and purified using a Ni-NTA column. NdmA-His(6) plus His(6)-NdmD catalyzed N(1)-demethylation of caffeine, theophylline, paraxanthine, and 1-methylxanthine to theobromine, 3-methylxanthine, 7-methylxanthine, and xanthine, respectively. NdmB-His(6) plus His(6)-NdmD catalyzed N(3)-demethylation of theobromine, 3-methylxanthine, caffeine, and theophylline to 7-methylxanthine, xanthine, paraxanthine, and 1-methylxanthine, respectively. One formaldehyde was produced from each methyl group removed. Activity of an N(7)-specific N-demethylase, NdmC, has been confirmed biochemically. This is the first report of bacterial N-demethylase genes that enable bacteria to live on caffeine. These genes represent a new class of Rieske oxygenases and have the potential to produce biofuels, animal feed, and pharmaceuticals from coffee and tea waste.

Characterization of isolated nitrogenase FeVco

The cofactors of the Mo- and V-nitrogenases (i.e., FeMoco and FeVco) are homologous metal centers with distinct catalytic properties. So far, there has been only one report on the isolation of FeVco from Azotobacter chroococcum. However, this isolated FeVco species did not carry the full substrate-reducing capacity, as it is unable to restore the N(2)-reducing ability of the cofactor-deficient MoFe protein. Here, we report the isolation and characterization of a fully active species of FeVco from A. vinelandii. Our metal and activity analyses show that FeVco has been extracted intact, carrying with it the characteristic capacity to reduce C(2)H(2) to C(2)H(6) and, perhaps even more importantly, the ability to reduce N(2) to NH(3). Moreover, our EPR and XAS/EXAFS investigations indicate that FeVco is similar to, yet distinct from FeMoco in electronic properties and structural topology, which could account for the differences in the reactivity of the two cofactors. The outcome of this study not only permits the proposal of the first EXAFS-based structural model of the isolated FeVco but also lays a foundation for future catalytic and structural investigations of this unique metallocluster.

Purification and Properties of Component B of 2,4,5-Trichlorophenoxyacetate Oxygenase from Pseudomonas cepacia AC1100

Pseudomonas cepacia AC1100 degrades 2,4,5-trichlorophenoxyacetate (2,4,5-T), an herbicide and chlorinated aromatic compound. Although some progress has been made in understanding 2,4,5-T degradation by AC1100 by molecular analysis, little is known about the biochemistry involved. Enzymatic activity converting 2,4,5-T to 2,4,5-trichlorophenol in the presence of NADH and O(inf2) was detected in cell extracts of AC1100. Phenyl agarose chromatography of the ammonium sulfate-fractionated cell extracts yielded no active single fractions, but the mixing of two fractions, named component A and component B, resulted in the recovery of enzyme activity. Component B was further purified to homogeneity by hydroxyapatite and DEAE chromatographies. Component B had a native molecular weight of 140,000, and it was composed of two 49-kDa (alpha)-subunits and two 24-kDa (beta)-subunits. Component B was red, and its spectrum in the visible region had maxima at 430 and 560 nm (shoulder), whereas upon reduction it had maxima at 420 (shoulder) and 530 nm. Each mole of (alpha)(beta) heterodimer contained 2.9 mol of iron and 2.1 mol of labile sulfide. These properties suggest strong similarities between component B and the terminal oxygenase components of the aromatic ring-hydroxylating dioxygenases. Component A was highly purified but not to homogeneity. The reconstituted 2,4,5-T oxygenase, consisting of components A and B, converted 2,4,5-T quantitatively into 2,4,5-trichlorophenol and glyoxylate with the coconsumption of NADH and O(inf2).

Assay methods and biological roles of labile sulfur in animal tissues

Sulfur is a chemically and biologically active element. Sulfur compounds in animal tissues can be present in two forms, namely stable and labile forms. Compounds such as methionine, cysteine, taurine and sulfuric acid are stable sulfur compounds. On the other hand, acid-labile sulfur and sulfane sulfur compounds are labile sulfur compounds. The sulfur atoms of labile sulfur compounds are liberated as inorganic sulfide by acid treatment or reduction. Therefore, the determination of sulfide is the basis for the determination of labile sulfur. Determination of sulfide has been performed by various methods, including spectrophotometry after derivatization, ion chromatography, high-performance liquid chromatography after derivatization, gas chromatography, and potentiometry with a sulfide ion-specific electrode. These methods were originally developed for the determination of sulfide in air and water samples and were then applied to biological samples. The metabolic origin of labile sulfur in animal tissues is cysteine. The pathways of cysteine metabolism leading to the formation of sulfane sulfur are discussed. Finally, reports on the physiological roles and pathological considerations of labile sulfur are reviewed.

Purification and characterisation of the NADH:acceptor reductase component of xylene monooxygenase encoded by the TOL plasmid pWW0 of Pseudomonas putida mt-2

The xylene monooxygenase system encoded by the TOL plasmid pWW0 of Pseudomonas putida catalyses the hydroxylation of a methyl side-chain of toluene and xylenes. Genetic studies have suggested that this monooxygenase consists of two different proteins, products of the xylA and xylM genes, which function as an electron-transfer protein and a terminal hydroxylase, respectively. In this study, the electron-transfer component of xylene monooxygenase, the product of xylA, was purified to homogeneity. Fractions containing the xylA gene product were identified by its NADH:cytochrome c reductase activity. The molecular mass of the enzyme was determined to be 40 kDa by SDS/PAGE, and 42 kDa by gel filtration. The enzyme was found to contain 1 mol/mol of tightly but not covalently bound FAD, as well as 2 mol/mol of non-haem iron and 2 mol/mol of acid-labile sulfide, suggesting the presence of two redox centers, one FAD and one [2Fe-2S] cluster/protein molecule. The oxidised form of the protein had absorbance maxima at 457 nm and 390 nm, with shoulders at 350 nm and 550 nm. These absorbance maxima disappeared upon reduction of the protein by NADH or dithionite. The NADH:acceptor reductase was capable of reducing either one- or two-electron acceptors, such as horse heart cytochrome c or 2,6-dichloroindophenol, at an optimal pH of 8.5. The reductase was found to have a Km value for NADH of 22 microM. The oxidation of NADH was determined to be stereospecific; the enzyme is pro-R (class A enzyme). The titration of the reductase with NADH or dithionite yielded three distinct reduced forms of the enzyme: the reduction of the [2Fe-2S] center occurred with a midpoint redox potential of -171 mV; and the reduction of FAD to FAD. (semiquinone form), with a calculated midpoint redox potential of -244 mV. The reduction of FAD. to FAD.. (dihydroquinone form), the last stage of the titration, occurred with a midpoint redox potential of -297 mV. The [2Fe-2S] center could be removed from the protein by treatment with an excess of mersalyl acid. The [2Fe-2S]-depleted protein was still reduced by NADH, giving rise to the formation of the anionic flavin semiquinone observed in the native enzyme, thus suggesting that the electron flow was NADH --> FAD --> [2Fe-2S] in this reductase. The resulting protein could no longer reduce cytochrome c, but could reduce 2,6-dichloroindophenol at a reduced rate.

Purification and properties of the NADH reductase component of alkene monooxygenase from Mycobacterium strain E3

Alkene monooxygenase, a multicomponent enzyme system which catalyzes the epoxidation of short-chain alkenes, is induced in Mycobacterium strain E3 when it is grown on ethene. We purified the NADH reductase component of this enzyme system to homogeneity. Recovery of the enzyme was 19%, with a purification factor of 920-fold. The enzyme is a monomer with a molecular mass of 56 kDa as determined by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It is yellow-red with absorption maxima at 384, 410, and 460 nm. Flavin adenine dinucleotide (FAD) was identified as a prosthetic group at a FAD-protein ratio of 1:1. Tween 80 prevented irreversible dissociation of FAD from the enzyme during chromatographic purification steps. Colorimetric analysis revealed 2 mol each of iron and acid-labile sulfide, indicating the presence of a [2Fe-2S] cluster. The presence of this cluster was confirmed by electron paramagnetic resonance spectroscopy (g values at 2.011, 1.921, and 1.876). Anaerobic reduction of the reductase by NADH resulted in formation of a flavin semiquinone.

Characterization of the FNR protein of Escherichia coli, an iron-binding transcriptional regulator

FNR is a transcriptional regulator mediating the activation or repression of a variety of Escherichia coli genes in response to anoxia. The FNR protein resembles CRP (the cyclic-AMP receptor protein) except for the presence of a cysteine-rich N-terminal segment which may form part of an iron-binding redoxsensing domain. The FNR protein was purified by a new procedure. It was monomeric (Mr = 30,000) and contained as much as 1.1 mol of iron per monomer when purified in the presence of added iron. This iron was associated with cysteine residues, because there was an inverse relation between iron content and titratable sulphydryl groups. Other physical and chemical properties are reported including evidence for a potential disulphide group or analogous modification. The interaction between FNR protein and target DNA appeared weak and non-specific in gel-retardation assays, but specific binding to the proposed DNA-binding site was shown for the first time in footprinting studies. A role for iron in FNR-mediated gene expression was confirmed by using cultures in which FNR was inactivated by growth in the presence of the specific chelator, ferrozine, but protected by ferrous iron.

4-Sulphobenzoate 3,4-dioxygenase. Purification and properties of a desulphonative two-component enzyme system from Comamonas testosteroni T-2

Cell-free extracts of Comamonas testosteroni T-2 grown in toluene-p-sulphonate/salts medium catalyse the conversion of p-sulphobenzoate (PSB) into protocatechuate and sulphite by an NADH-requiring and Fe2(+)-activated dioxygenase. Anion-exchange chromatography of extracts yielded red (A) and yellow (B) protein fractions, both of which were necessary for dioxygenative activity. Further purification of each fraction by hydrophobic interaction chromatography and gel filtration led to two homogeneous protein components (A and B), which together converted 1 mol each of PSB, O2 and NADH into 1 mol each of protocatechuate, sulphite and, presumably, NAD+. The system was named 4-sulphobenzoate 3,4-dioxygenase (PSB dioxygenase system). Monomeric component B (Mr 36,000) was determined to be a reductase that contained 1 mol of FMN and about 2 mol each of iron and inorganic sulphur per mol. This component transferred electrons from NADH to the oxygenase component (A) or to, e.g., cytochrome c. Homodimeric component A (subunit Mr 50,000) of the PSB dioxygenase system contained one [2Fe-2S] centre per subunit and its u.v.-visible-absorption spectrum corresponded to a Rieske-type iron-sulphur centre. The requirement for activation by iron was interpreted as partial loss of mononuclear iron during purification of component A. Component A could be reduced by dithionite or by NADH plus catalytic amounts of component B. The PSB dioxygenase system displayed a narrow substrate range: none of 18 sulphonated or non-sulphonated analogues of PSB showed significant substrate-dependent O2 uptake. The physical properties of the PSB dioxygenase system resemble those of other bacterial multi-component dioxygenase, especially phthalate dioxygenase. However, it differs from most characterized systems in its overall reaction; the product is a vicinal diphenol, and not a dihydrodiol.

Purification and characterization of the pyruvate-ferredoxin oxidoreductase from Clostridium acetobutylicum

The pyruvate-ferredoxin oxidoreductase from Clostridium acetobutylicum was purified to homogeneity and partially characterized. A 9.2-fold purification was achieved in a three step purification procedure: ammonium sulfate fractionation, chromatography on Phenyl Sepharose and on Procion Blue H-EGN12. The pure enzyme exhibited a specific activity of 25 U/mg of protein. Homogeneity of the pyruvate-ferredoxin oxidoreductase was confirmed by native polyacrylamide gel electrophoresis and sodium dodecylsulfate (SDS)-polyacrylamide gel electrophoresis. The molecular weight was determined to be 123,000/monomer. The subunit composition of the native enzyme could not be determined because of the instability of the pure enzyme. The pyruvate-ferredoxin oxidoreductase is sensitive to oxygen and dilution during purification. The dilution inactivation could be partially overcome by the addition of 300 microM coenzyme A or 50% ethyleneglycol. A thiamine pyrophosphate content of 0.39 mol per mol of enzyme monomer was found, the iron and sulfur content was 4.23 and 0.91, respectively. The pH-optimum was at pH 7.5 and the temperature optimum was at 60 degrees C. Kinetic constants were measured in the forward reaction. The apparent Km for pyruvate and coenzyme A were 322 microM and 3.7 microM, respectively. With 2-ketobutyrate the pyruvate-ferredoxin oxidoreductase showed 12.5% of the activity compared to pyruvate. No activity was found with 2-ketoglutarate. Ferredoxin from Clostridium pasteurianum could be used as physiological electron acceptor.

Purification and properties of membrane-bound hydrogenase isoenzyme 1 from anaerobically grown Escherichia coli K12

Hydrogenase isoenzyme 1 from the membrane fraction of anaerobically grown Escherichia coli has been purified to near homogeneity. The preparation involved dispersion of the membrane fraction with deoxycholate followed by ammonium sulphate precipitation, ion-exchange, hydroxyapatite and gel filtration chromatography steps. The enzyme was assayed by quantification of the H2:benzyl viologen oxidoreductase activity immunoprecipitated by a non-inhibitory antiserum specific for the enzyme. The enzyme constituted about 8% of the hydrogenase activity found in the detergent-dispersed membranes, the remainder being attributable to hydrogenase isoenzyme 2. Isoenzyme 1 was purified 130-fold and the specific activity of the final preparation was 10.6 mumol benzyl viologen reduced min-1 (mg protein)-1 (H2:benzyl viologen oxidoreductase). The final preparation contained polypeptides of apparent Mr 64,000, 31,000 and 29,000. Antibodies were raised both to the final preparation and to immunoprecipitation arcs containing hydrogenase isoenzyme 1, excised from crossed immunoelectrophoresis plates. The former cross-reacted with all three polypeptides in the enzyme preparation but the latter recognised only the Mr-64,000 polypeptide. Immunological analysis revealed that the polypeptides of apparent Mr 31,000 and 29,000 are fragments of a single polypeptide of Mr 35,000 which is present in the detergent-dispersed membranes. The fragmentation of the Mr-35,000 polypeptide during the preparation correlates with a change in the electrophoretic mobility of the enzyme. A similar electrophoretic mobility change was observed, accompanied by cleavage of the Mr-35,000 polypeptide to one of 32,000 when the enzyme was analysed after exposure of detergent-dispersed membranes to trypsin. The enzyme in the detergent-dispersed membranes consists minimally of two subunits of Mr 64,000 and two subunits of Mr 35,000. It contained 12.2 mol Fe and 9.1 mol acid-labile S2-/200,000 g enzyme. The enzyme, purified from bacteria grown in the presence of 63Ni, was found to contain 0.64 (+/- 0.20) mol Ni/200,000 g enzyme. A constant ratio of 63Ni immunoprecipitated to hydrogenase isoenzyme 1 activity immunoprecipitated by antiserum specific for the enzyme was observed during the preparation, consistent with Ni being part of the enzyme. The enzyme has a low Km for H2 (2.0 microM) in the H2:benzyl viologen oxidoreductase assay. It catalyses H2 evolution employing reduced methyl viologen as electron donor. It is inhibited reversibly by CO and irreversibly by N-bromosuccinimide.

Purification, amino-acid sequence and some properties of the ferredoxin isolated from Bacillus acidocaldarius

Ferredoxin was isolated from the aerobic, thermophilic and acidophilic bacterium Bacillus acidocaldarius and its sequence of 78 amino acids completely determined by automated Edman degradation of the protein and of peptides derived from chemical cleavage between aspartic acid and proline and from enzymatic digestions. The optical spectrum of the oxidized protein has a broad maximum around 400 nm. The ferredoxin is thermostable: its absorbance begins to decrease only at incubation over 71 degrees C. The number of iron and inorganic sulphur atoms per molecule was determined to be 5.3 and 5.0, respectively. The calculated molar extinction coefficient was 23 000 M-1 X cm-1, the molecular mass of the apoferredoxin 8 872 Da. Contrary to all expectations, the sequence of B. acidocaldarius ferredoxin shows very little homology to that of B. stearothermophilus but closely resembles that of Thermus thermophilus.

Purification and properties of the soluble hydrogenase from Desulfovibrio desulfuricans (strain Norway 4)

A soluble hydrogenase has been isolated from Desulfovibrio desulfuricans (strain Norway 4) grown on Postgate's medium. The enzyme differs significantly from a membrane-bound hydrogenase previously purified from the same organism grown on Starkey's medium. The enzyme consisted of two subunits of 56 kDa and 29 kDa compared with masses of 60 kDa and 27 kDa for the membrane-bound enzyme. Analysis of preparations of the soluble enzyme by various methods gave values of 5-10 iron atoms, 6 labile sulphur atoms and 0.45-0.8 nickel atom per molecule. The enzyme was unusual in that it contained selenium, in quantities equivalent to nickel. The highly purified active enzyme produced no electron-spin-resonance (ESR) signals in the oxidized state. ESR signals due to a [3Fe-xS] cluster and nickel were observed only in some of the less active fractions of the enzyme, demonstrating that neither of these ESR-detectable components is a prerequisite for hydrogenase activity. Treatment of D. desulfuricans (Norway) cells with EDTA released a minor fraction with hydrogenase activity, which might indicate the presence of a periplasmic enzyme.

Comparison of the properties of two hydrogenases from the photosynthetic bacterium Chromatium vinosum

Chromatium vinosum hydrogenases I and II were purified to specific activities of 9.6 and 28.0 units/mg protein, respectively. They have the same isoelectric point (pI = 4.1), and their visible spectra are typical of iron-sulfur proteins. Hydrogenase II in general was more stable than hydrogenase I. Both enzymes lost their activities slowly during storage in air, and this inactivation was more apparent in preparations of hydrogenase I. Bovine serum albumin helped to stabilize hydrogenase I against thermal and storage inactivation. The pH optima of H2-evolution activity of hydrogenases I and II were 7.4 and 5.4, respectively. Neither enzyme was able to evolve H2 from reduced ferredoxins as the sole electron carrier, but ferredoxins had an effect on the activity with methyl viologen as carrier to hydrogenase I. None of the natural compounds tested was able to serve as a physiological donor for H2 production. Hydrogenase I was more susceptible than hydrogenase II to inhibition by heavy metal ions and other enzyme inhibitors. Both enzymes were reversibly inhibited by CO with Ki values of 12 and 6 Torr for hydrogenase I and II, respectively. Hydrogenase I was more sensitive to denaturation by urea and guanidinium chloride while hydrogenase II was more susceptible to sodium dodecyl sulfate. Both enzymes were rapidly and irreversibly inactivated by dimethyl sulfoxide. Hydrogenase I evolved H2 from methyl viologen and ferredoxin photoreduced by chloroplasts. The enzymes differed in their iron and acid-labile sulfur contents.

Semi-micro methods for analysis of labile sulfide and of labile sulfide plus sulfane sulfur in unusually stable iron-sulfur proteins

Details are provided for a reproducible procedure for determination of labile sulfide in iron-sulfur (Fe-S) proteins in the range of 1 to 3 nmol. Analyses are also presented on the most stable Fe-S protein so far reported. In this case denaturation with guanidine.HCl was used in the presence of dithiothreitol. The values obtained then also include any sulfane sulfur (S0) present.

Purification and properties of the membrane-bound by hydrogenase from Desulfovibrio desulfuricans

The membrane-bound hydrogenase from the anaerobic sulphate-reducing bacterium Desulfovibrio desulfuricans (Norway strain) has been purified to homogeneity, with an overall 80-fold purification and a specific activity of 70 mumol of H2 evolved/min per mg of protein. The hydrogenase had a relative molecular mass of 58 000 as determined by gel filtration and was estimated to contain six iron atoms and six acid-labile sulphur groups per molecule. The absorption spectrum of the enzyme was characteristic of an iron-sulphur protein. The E400 and E280 were 28 500 and 109 000 M-1.cm-1 respectively. The e.s.r. of the oxidized protein indicated the presence of [4Fe-4S]3+ or [3Fe-3S]3+, and another paramagnetic centre, probably Ni(III). The hydrogenase was inhibited by heavy-metal salts, carbon monoxide and high ionic strength. However, it was resistant to inhibition by thiol-blocking and metal-complexing reagents. N-Bromosuccinimide totally inhibited the enzyme activity at low concentrations. The enzyme was stable to O2 over long periods and to high temperatures. It catalyses both H2-evolution and H2-uptake with a variety of artificial electron carriers. D. desulfuricans cytochrome C3, its natural electron carrier, had a high affinity for the enzyme (Km = 2 microns). Rate enhancement was observed when cytochrome C3 was added to Methyl Viologen in the H2-evolution assay. The pH optimum for H2-evolution was 6.5.

Electron transport to nitrogenase in Klebsiella pneumoniae: purification and properties of the nifJ protein

In Klebsiella pneumoniae, the physiological electron flow to nitrogenase involves specifically, in addition to nitrogenase reductase, the products of the nifF and nifJ genes. The J protein was purified to homogeneity and was found to be an iron-sulfur protein devoid of molybdenum. In its native state, the J protein is a dimer of Mr about 245 000, made up of two subunits of the same molecular weight. It contains about 30 mol iron and 24 mol labile sulfur/mol protein. The addition of J protein to crude extracts of a nifJ mutant reestablishes pyruvate-supported acetylene-reducing activity. This activity is further enhanced by addition of pure nitrogenase (Kp1). Based on its physical properties, the J protein is probably an oxidoreductase whose physiological role might be to transfer electrons from a metabolic donor to the F protein. In addition, another protein whose activity is also dependent on the nifJ gene seems to be required for the formation of a fully active Kp1.

Purification and characterization of NADH oxidase from membranes of Acholeplasma laidlawii, a copper-containing iron-sulfur flavoprotein

1. NADH oxidase was extracted from the membranes of Acholeplasma laidlawii with buffer containing 3% Triton X-100 and subsequently purified by several chromatographic steps. The final preparation was essentially homogeneous as judged by gel electrophoresis under nondenaturing conditions. 2. The enzyme appears to be a copper-containing iron-sulfur flavoprotein (FMN:CU:Fe:labile S = 1:1:6:6). The enzyme, containing a high fraction of hydrophobic amino acids, is composed of three subunits of molecular weight 65 000, 40 000 and 19 000. 3. When oxygen is used as electron acceptor the purified enzyme demonstrates a specific activity of 58.0 IU/mg of protein and catalyzes the formation of H2O2 in nearly stoichiometric amount. The apparent Km value for NADH is estimated to be 0.4 mM (pH 7.4). NADPH cannot serve as a substrate for the enzyme. In addition to the NADH oxidase activity, the enzyme is able to catalyze electron transfer from NADH to various other electron acceptors (ferricyanide, dichloroindophenol, cytochrome c). Metal-chelating agents and mercurials are shown to inhibit the activity of the enzyme. 4. From electron paramagnetic resonance and optical absorption measurements evidence was obtained that the flavin semiquinone radical in the NADH oxidase has a high air-stability, and that the flavin shuttles between the fully reduced and the semiquinone state upon electron transport from NADH to the electron acceptors. Inhibition of the NADH oxidoreductase activities by superoxide dismutase indicates that O-2 serves as an intermediate in the electron transfer from NADH to all electron acceptors used in this work. In addition to electron transfer via the superoxide radical O-2, an alternative pathway probably involving Fe-S centers is operative. From these results and literature data we present a reaction scheme for electron transport from NADH to the various electron acceptors.

Characterization of the respiratory NADH dehydrogenase of Escherichia coli and reconstitution of NADH oxidase in ndh mutant membrane vesicles

Highly purified preparations of the cholate-solubilized respiratory NADH dehydrogenase, isolated from genetically amplified Escherichia coli strains [Jaworowski, A., Campbell, H. D., Poulis, M. I., & Young, I. G. (1981) Biochemistry 20, 2041-2047], have been characterized. Enzyme preparations were shown to contain 70% (w/w) lipid, predominantly phosphatidylethanolamine. One mol of noncovalently bound FAD and approximately 1 mol of ubiquinone/mol of enzyme subunit were detected. The purified enzyme was shown to contain only low levels of Fe and acid-labile S, indicating the absence of iron-sulfur clusters. No Cu, Mo, W, or covalently bound P was detected, and no evidence for other chromophores was obtained from visible and ultraviolet absorption spectra of the purified enzyme or of the delipidated polypeptide prepared by gel filtration in sodium dodecyl sulfate. Protein chemical studies verified that the enzyme consists of a single polypeptide species of Mr 47 000, and the N- and C-terminal cyanogen bromide peptides were identified. The pure enzyme was shown to reconstitute membrane-bound, cyanide-sensitive NADH oxidase activity in membrane vesicles prepared from ndh mutant strains.

Purification, some properties and amino acid sequence of Thermus thermophilus HB8 ferredoxin

A stable ferredoxin was purified in a crystalline form from an aerobic, thermophilic bacterium, Thermus thermophilus HB8. The molecular weight of the protein was determined to be 10500 by gel-filtration on Sephadex G-75 and to be 10200 by the sedimentation equilibrium method. The number of iron and acid labile sulfur atoms per mol was determined to be 6.3 and 6.4, respectively. The optical absorption spectrum of the ferredoxin has a broad maximum around 400 nm. The ferredoxin was so thermostable that its absorbance at 400 nm did not decrease after a 45-min incubation at 65 degrees C. The primary structure of the ferredoxin consisting of 78 amino acids was determined by sequence analysis of peptides obtained from a tryptic digest of the S-carboxymethylated ferredoxin and from a Staphylococcus aureus V8 protease digest of the S-aminoethylated derivative. The distribution of cysteine residues and the amino acid sequence around the cysteine residues are very similar to those of Mycobacterium smegmatis ferredoxin.

Purification and some properties of a hitherto-unknown enzyme reducing the carbon-carbon double bond of alpha, beta-unsaturated carboxylate anions

2-Enoate-reductase, a previously unknown soluble enzyme is present in Clostridium kluyveri and another Clostridium species growing on (E)-2-butenoate. From the latter the reductase was purified 88-fold with an overall yield up to 74%. The enzyme was pure as judged by polyacrylamide gel electrophoresis with and without sodium dodecyl sulphate as well as by isoelectric focusing. The purification of the enzyme was performed in the presence of (E)-2-methyl-2-butenoate as substrate to keep the enzyme in the oxidized state and under anaerobic conditions. The purification procedure included an ammonium sulphate precipitation, chromatography on DEAE-Sepharose CL-6B, hydroxylapatite and Sepharose CL-6B. The enzyme reduces different alpha,beta-unsaturated carboxylate anions such as (E)-2-butenoate, (E)-2-methyl-2-butenoate, (E)-cinnamate and probably many others in a NADH-dependent reaction to the saturated carboxylate anions. Fumarate, 3-phenyl-2-propinate, 2-enoyl-methyl and CoA esters proved not to be substrates for the purified reductase. NADPH does not act as an electron donor. The enzyme was shown to have a molecular weight of about 450,000 by gel chromatography. It consists of subunits with a molecular weight of 78,000. Per subunit about 1 FAD, 3.5--3.8 atoms of iron and 4.0 labile sulphur atoms have been found indicating a conjugated iron-sulphur flavo-protein. Copper could not be detected. The isoelectric point was 8.4. As shown by absorption spectroscopy the enzyme can be reduced by NADH and reoxidized with dichloroindophenol, hexacyanoferrate III, oxygen and substrates. Addition of 8 mol p-hydroxymercuribenzoate to 1 mol subunit completely destroyed the activity of the reductase. So far no physiological role of the enzyme is known.

Two plant-type ferredoxins from a blue-green alga, Nostoc verrucosum

Two plant-type ferredoxins were isolated and purified from a blue-green alga, Nostoc verrucosum. They were separable by chromatography on a DEAE-cellulose column. The slow-moving band was designated ferredoxin I (Fd I) and the fast-moving band was ferredoxin II (Fd II). The ratio of the yield of ferredoxins I and II was about 1 : 0.84. Both ferredoxins had absorption spectra similar to those of plant-type ferredoxins. Two atoms of non-heme iron and two of labile sulfur were found per mol of both ferredoxin I and ferredoxin II. Their molecular weights were identical and estimated to be about 18 000 by a gel filtration method. The biochemical activities of these Nostoc ferredoxins were studied: the NADP photoreduction activity on one hand and the NADP-cytochrome c reductase activity on the other.